Start-run plural cathode structure



Nov. 2, 1965 J. F. WAYMOUTH 3,215,881

START-RUN PLURAL GATHODE STRUCTURE I Filed Aug. 20, 1962 INVENTOR.

United States Patent 3,215,881 START-RUN PLURAL CATHQDE STRUCTURE John F. Waymouth, Marhlehead, Mass., assignor to Sylvania Electric Products Inc., Wilmington, DeL, a corporation of Delaware Filed Aug. 20, 1962, Ser. No. 217,948 12 Claims. (Cl. 3131tl9) This invention relates to cathode structures for electric discharge lamps such as fluorescent lamps and more particularly to a two cathode structure which substantially increases the useful life of fluorescent lamps under intermittent use.

Fluorescent lamps are often used in services where they are turned on and oil frequency. In designing a lamp for maximum life under such conditions, the conventional single filament cathode at each end of a conventional tubular envelope has typically represented a compromise which attempted to satisfy the widely diverse conditions existing during starting and during continuous arc operation. In starting a fluorescent lamp with a rapid-start type of ballast, the filament heating current and the arc supporting potential are applied simultaneously. Until each of the cathodes is heated to a temperature at which substantial electron emission is possible, each entire cathode structure is covered with a glow discharge which subjects the relatively fragile filamentary cathode to a damaging ion bombardment. This effect is particularly pronounced in fluorescent lamps in which the fill gas is largely neon because the lighter mass of the neon atoms offer less protection from ion bombardrnent than the more conventional argon or krypton fill gas. When the cathode reaches emitting temperature the illuminating arc is established and the ion bombardment is greatly reduced.

To minimize the total damage done to the cathode by ion bombardment, it is of course desirable that the cathode reach operating temperature as quickly as possible. To heat quickly, the cathode should have low heat capacity or thermal inertia and should draw enough power to produce a relatively high operating or equilibrium temperature so that the cathode begins to emit a substantial quantity of electrons while its temperature is still rising rapidly.

These characteristics necessary for rapid heating are, however, incompatible with a long life for the cathode under continuous are operating conditions. For a long life during continuous operation, the cathode should have a large emitting surface, a large reservoir of active eletron emissive material and should operate at as low a temperature as possible. Each of these latter characteristics tend to slow the initial heating of a cathode.

It has been proposed to use at each end of the fluorescent envelope a cathode structure including a pair of cathodes of substantially identical starting and operating characteristics so that each contributes equally to the electron stream through the lamp thereby distributing the deterioration. While the two cathodes will last longer than one, each cathode still involves a compromise between good starting and good operating characteristics.

The object of the present invention is to avoid the compromise of prior fluorescent lamps and provide a cathode structure having both good starting and good operating characteristics and which therefore can withstand many more starts than conventional lamps.

In this new cathode structure there are provided two filamentary cathodes, one of which is adapted for rapid heating and the other of which is adapted for long life under continuous operation, and these cathodes are arranged so that an are started on said one cathode will transfer to said other cathode as soon as said other cathode reaches electron emitting temperature. The

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arrangement of the cathodes provided is such that the starting cathodes are spaced from the operating cathode in a direction opposite the electron flow from the cathode structure. The negative resistance characteristic of the are then insures that the transfer will occur as soon as the operating cathode is at a temperaturecapable of supporting the arc discharge. Thus, according to the invention, the cathode structure comprises a first filamentary cathode having a substantial reservoir of electron emissive material and a second filamentary cathode of low heat capacity, said second cathode being spaced from first cathode in a direction directly increasing the discharge path length.

For the purpose of illustration a preferred embodiment of the invention is shown in the accompanying drawing in which FIG. 1 is a side view of a cathode structure, FIG. 2 is a side view partially in section of a fluorescent lamp incorporating a cathode structure according to FIG. 1.

In FIG. 1 the cathode structure is shown mounted on an end wall member 10 which is adapted. to seal the end of a tubular fluorescent lam-p envelope 40 as shown in FIG. 2. As is customary in the art, the tube is provided with similar cathodes at each end, the interior is coated with a phosphor 42. A small quantity of mercury and approximately %-100% of neon and 20%()% of argon at low pressure is sealed therein.

The member 10 includes an aperture 12 through which the envelope 40 may be evacuated by means of the usual exhaust tube 14. A pair of lead-in wires 16 and 18 pass through the member 10 in generally parallel relation and so that their ends project equal distances into the lamp. The opposite ends of the lead-in wires are connected to contact pins 44 in the base 46 of the lamp.

The lead-in wires support two filamentary cathodes 20 and 22 which extend generally parallel to each other and perpendicular to the lead-in wires. Preferably the lead-in wires also support a pair of radiational cooling plates 24 and 26 which tend to control the mercury vapor pressure in the lamp in a known manner. Each of these plates is welded to one of the lead-in wires, as at tabs 28 and 30 respectively, and is insulated from the remaining wire by a ceramic bushing 32.

By way of example, the filamentary cathodes 20 and 22 can be of coiled tungsten wire, preferably of the coiled coil or triple coiled types well known in the art, and carry a quantity of electron emitting substances such as the usual alkaline earth oxides, preferably with the addition of a small quantity of zirconium oxides. According to the invention, however, the operating cathode 2G is adapted, by the choice of its design parameters according to well known principles, to operate at a relatively low equilibrium temperature and is provided with a substantial reserve of active electron emitting material. The operating cathode 20 is, in other words, adapted for maximum life under continuous operating conditions. For example, the operating cathode 20 can carry 30-50 milligrams of electron emissive material having about two square centimeters of emitting surface area and have a filament drawing enough power to provide operation in the approximate range of 800 to 830 C. Such a cathode requires about six seconds to heat to emitting temperature. For operation at the conventional filament volt-age of 3.8 a resistance of approximately 2 ohms provides appropriate heating power.

The starting cathode 22 on the other hand carries but a light coating of electron emitting material, for example about 5 milligrams providing an emitting surface of about one quarter of a square centimeter, and. its filament is adapted to draw enough power to maintain its equilibrium temperature in the approximate range of 860 to 900 C. Such a cathode may be made to reach electron emitting and are sustaining operation in about 1 /2 seconds. The appropriate heating resistance is approximately lOohms at 3.8 volts.

It is an important feature of the invention that the cathode 22, the starting cathode, is located further from the center of the lamp than the operating cathode 20. This construction permits the illuminating arc to shift from the starting to the operating cathode when that latter cathode reaches electron emitting temperature. Because of its negative resistance characteristic, the arc discharge always tends to operate in such a manner that the total potential difference across the discharge is at a minimum and, since the shift from the starting cathode 22 to the operating cathode 20 shortens the arc column thereby reducing the arc drop, this shift will always tend to occur when the operating cathode 20 reaches emission. The construction illustrated further facilitates this shift by providing an uninsulated metallic connection between the two cathodes within the envelope, the two heating filaments being electrically in parallel across the lead-in wires 16 and 18.

With fluorescent tubes of a power rating of 25 watts per foot, an axial spacing between the filamentary cathodes 20 and 22 of about inch to 1 inch is appropriate. The spacing in any case should be suflicient so that each cathode is outside of the negative glow of the other during the corresponding modes of operation.

While a difference in equilibrium temperature between the two cathodes is desirable according to the invention for hastening the rise in temperature of the starting cathode 22 to emitting temperature, it has been found that, for cathodes of generally similar materials, that the temperature difference should not exceed 100 C. if a reliable transfer of the arc is to be achieved.

The sequence of operation during starting is as follows: Upon application of filament heating current and are producing voltage to the lamp, a glow discharge uniformly covers all of the conducting parts of the cathode structure and subjects them to severe ion bombardment. After approximately a second and a half, when the starting cathode 22 has heated to emitting temperature, the discharge converts to an arc on the starting cathode 22. The establishment of an are also greatly reduces the ion bombardment of the operating cathode 20 even though that cathode has not yet reached electron emitting temperature. When the operating cathode 20 does reach operating temperature, some four or five seconds later, the arc discharge transfers from the starting cathode 22 to the operating cathode 20, lowering the arc drop. Thereafter the starting cathode draws negligible power, and the operating cathode draws the rated wattage. That is, the starting cathode 22 alone draws substantially all the arc discharge power until an abrupt transfer of the arc, and then the operating cathode 20 draws substantially all the arc discharge power.

Fluorescent lamps built according to the present invention and operated on a 2 /2 minute on2 /2 minute off cycle survive at least double the number of starts that an equally rated lamp can survive. While this performance clearly demonstrates the improved starting characteristics of the lamp of present invention, a related test shows its superiority in customary use. Lamps manufactured according to the invention have been operated on a 3 hour n20 minute off cycle for 9000 hours, double the expected life of prior lamps. These are particularly significant advances in primarily neon filled lamps.

It should be understood that this disclosure is for the purpose of illustration only and that the present invention includes all modifications and equivalents falling within the scope of the appended claims.

I claim:

1. A fluorescent lamp comprising an elongate tubular envelope, a coating of phosphor on the interior surface thereof, an end wall member at each end of the envelope, a pair of generally parallel lead-in wires extending inwardly from each end wall member axially of the envelope, two axially spaced filamentary cathodes extending between each said pair of lead-in wires approximately parallel to each other and perpendicular to the length of the envelope, both cathodes being adapted to be heated by current through said lead-in wires, the cathodes nearest the center of the envelope having substantial reservoirs of electron emissive material and being adapted for operation at low are sustaining temperatures, and said cathodes nearest the center of the envelope having substantially higher thermal inertia and substantially lower resistance than the cathodes nearest the end walls, the cathodes nearest the end walls being adapted for operation at equilibrium temperatures higher than the cathodes nearer the center of the envelope.

2. A cathode structure according to claim 1 in which the difference in equilibrium temperatures of the two cathodes is less than C.

3. An electrical discharge lamp comprising an envelope providing a discharge path, an electrode structure at each end of said path, at least one of said electrode structures including a starting cathode and an operating cathode axially spaced therefrom, electrical connections for supplying heating and discharge current to both said cathodes, the operating cathode being adapted for maximum operating life, the starting cathode having a heat capacity substantially lower and an ohmic resistance substantially higher than that of the operating cathode for substantially faster heating to are emitting temperature than the operating cathode, so that said operating cathode is protected from ion bombardment while it is heating to emitting temperature, the starting cathode being located at one end of the path and the operating cathode being located adjacent said starting cathode and between said starting cathode and the other end of the path, whereby the arc transfers from said starting cathode to said operating cathode when the latter reaches arc emitting temperature.

4. An electrical discharge lamp according to claim 3 wherein said operating cathode has a substantially greater emitting area than said starting cathode.

5. An electrical discharge lamp according to claim 3 wherein said starting cathode is adapted to operate at a temperature within approximately 30 C. to 100 C higher than the emitting operating cathode.

6. In a fluorescent lamp operating at a rated power to produce an illuminating arc discharge between a cathode structure and a spaced electrode, a cathode structure comprising a starting and an operating filamentary cathode, each cathode having an electron emissive coating and an electrical resistance or heating the cathode to electron emission temperature, and electrical connections for supplying heating and discharge current thereto, the starting cathode having a low heat capacity relative to the operating cathode, and the starting cathode being axially spaced from the operating cathode in a direction directly increasing the length of arc discharge, and the resistance and heat capacity of the starting cathode being proportioned to hold the starting electrode at an equilibrium temperature higher than that of the operating cathode, whereby the starting cathode alone establishes and maintains the arc discharge while the operating cathode heats itself to emission temperature thereby substantially to reduce ion bombardment damage to the operating cathode, whereafter substantially the entire arc discharge transfers to the operating cathode.

7. A cathode structure according to claim 6 in which the equilibrium temperature of the operating cathode is within 100 lower than that of the starting cathode.

8. A cathode structure according to claim 7 wherein the equilibrium temperature of the operating cathode is 800 to 830 C. and the equilibrium temperature of the starting cathode is 860 to 900 C.

9. A cathode structure according to claim 6 wherein the starting and operating cathodes are connected in parallel electrically.

10. A cathode structure according to claim 6 wherein the spacing between the starting and operating cathodes is from inch to one inch such that each cathode is outside the negative glow of the other.

11. A cathode structure for an electric discharge lamp operating at a rated power to produce an illuminating arc discharge between said cathode structure and a spaced electrode, said cathode structure comprising a filamentary starting cathode, a filamentary operating cathode, each cathode having a reservoir of electron emissive material, and electrical connections for supplying heating current and discharge current to said cathodes, the starting cathode having an electrical resistance proportioned to heat the starting electrode to apredetermined emission temperature, the operating cathode having a substantially greater amount of said emissive material than the starting cathode and having an electrical resistance proportioned to heat the operating cathode to an emission temperature less than but of the same order as said predetermined emission temperature of the starting cathode, the operating cathode having a substantially higher thermal inertia and a substantially lower ohmic resistance than the starting cathode, and the entire starting cathode being axially spaced from the operating cathode in a direction directly increasing the length of discharge, whereby an initial arc discharge may be established by relatively rapid heating of the starting cathode and maintained solely by the starting cathode until the operating cathode is heated to its emission temperature, whereupon the arc discharge transfers to the operating cathode which then draws the rated power, and the starting cathode draws a negligible amount of power.

12. An electric discharge device comprising an envelope, spaced electrode structures defining an arc discharge path therebetween in said envelope, at least one of said electrode structures comprising a filamentary starting cathode and a filamentary operating cathode axially spaced from the starting cathode toward the other electrode structure, and electrical conductors for supplying heat and discharge current to both said cathodes, both cathodes being adapted for resistive heating, the operating cathode being adapted for maximum life at are supporting temperature, the operating cathode having a substantially higher thermal inertia and a substantially lower ohmic resistance than the starting cathode, and the starting cathode having a lower heat capacity so as to heat more rapidly to arc discharge supporting temperature relative to the operating cathode, thereby to protect the operating cathode from ion bombardment until the operating cathode is heated to arc discharge supporting temperature.

References Cited by the Examiner UNITED STATES PATENTS 3/38 Hunt 313-208 X 12/38 Perrott 3l3207 

1. A FLUORESCENT LAMP COMPRISING AN ELONGTE TUBULAR ENVELOPE, A COATING OF PHOSPHOR ON THE INTERIOR SURFACE THEREOF, AN END WALL MEMBER AT EACH END OF THE ENVELOPE, A PAIR OF GENERALLY PARALLEL LEAD-IN WIRES EXTENDING INWARDLY FROM EACH END WALL MEMBER AXIALLY OF THE ENVELOPE, TWO AXIALLY SPACED FILAMENTARY CATHODES EXTENDING BETWEEN EACH SAID PAIR OF LEAD-IN WIRES APPROXIMATELY PARALLEL TO EACH OTHER AND PERPENDICULAR TO THE LENGTH OF THE ENVELOPE, BOTH CATHODES BEING ADAPTED TO BE HEATED BY CURRENT THROUGH SAID LEAD-IN WIRES, THE CATHODES NEAREST THE CENTER OF THE ENVELOPE HAVING SUBSTANTIAL RESERVOIRS OF ELECTRON EMISSIVE MATERIAL AND BEING ADAPTED FOR OPERATION AT LOW AN SUSTAINING TEMPERATURES, AND SAID CATHODES NEAREST THE CENTER OF THE ENVELOPE HAVING SUBSTANTIALLY HIGHER THERMAL INEERTIA AND SUBSTANTIALLY LOWER RESISTANCE THAN THE CATHODES NEAREST THE END WALLS THE CATHODES NEAREST THE END WALLS BEING ADAPTED FOR OPERATION AT EQUILLIBRIUM TEMPERATURES HIGHER THAN THE CATHODES NEARER THE CENTER OF THE ENVELOPE. 